Pool-type discharge device



Oct. 10, 1939. L. D. MILES 2,175,908

POOLTYPE DISCHARGE DEVICE Filed Feb. 23, 1938 Inventor: Lawrence D. M Hes byJV A His Attorney.

Patented Oct. 10, 1939 UNITED STATES PATENT OFFICE POOL-TYPE DISCHARGE DEVICE of New York Application February 23, 1938, Serial No. 191,855

2 Claims.

This is a continuation in part of my application Serial No. 74,647, filed April 16, 1936. The invention described and claimed herein relates particularly to discharge devices of the type which employ a pool-type cathode, such as a cathode constituted of mercury.

Single anode rectifiers of this character are now available which are adapted to be rendered conductive at desired intervals in response to a controlling influence, transition from the nonconductive to the conductive state being accomplished by means of a cyclically operable makealive device. One type of make-alive device, for example, is disclosed in the Slepian and Ludwig 1 Patent No. 2,069,283 as comprising an electrode of semi-conducting material arranged in permanent contact with the cathode surface. Another form, involving the use of a shielded holding arc assembly, is described and claimed in my 20 copending application Serial No. 168,197, filed October 9, 1937, and assigned to the same assignee as the present invention.

Because of their high current-carrying capacity, discharge devices controlled in this fash- 25 ion have found wide application in power fields, especially in low voltage installations. At higher voltages, however, they display an inherent liability to arc-back which makes their operation uncertain and requires the use of considerable 30 protective equipment. It is an object of the present invention to provide means whereby the occurrence of arc-back may be substantially avoided even at relatively high operating voltages.

35 In accordance with a preferred embodiment of the invention a striking improvement in this respect is obtained by providing a series of cooperating baifles between the anode and cathode. These are preferably arranged so that a relatively tortuous path is provided for the discharge current and so that the anode is effectively shielded from direct exposure to the cathode surface. Since the use of a make-alive element tends to confine the discharge to a restricted region of the cathode surface, a baflie relationship can be established which is effective to minimize projection of cathode material into the vicinity of the anode. This and the other effects of multiple bafiling reduce greatly the possibility of areback occurrence.

The features of novelty which I desire to protect herein will be pointed out with particularity in the appended claims. The invention itself, together with further advantages thereof, may best i be understood by reference to the following description taken in connection with the drawing, in which Fig. 1 comprises a sectional View of a discharge device suitably embodying the invention; Fig. 2 is a transverse section taken on line 2-2 of Fig. 1; Fig. 3 is a view showing in sec- 5 tion the essential elements of a modified embodiment of the invention, and Figs. 4 and 5 are fragmentary detail views, respectively in section and in plan, of a particular element of the structure of Fig. 3.

Referring particularly to Fig, l I have shown a sealed evacuated envelope I, comprising a glass cylindrical portion provided at its upper end with a reduced portion which is closed by a reentrant cap 2 peripherally sealed to the glass. It should be noted that this cap as well as other elements referred to in the following as being sealed directly to glass may suitably comprise one of the alloys described in an article by Dr.

A. W. Hull and Mr. E. E. Burger appearing in Physics, vol. 5, No, 12, pages 384 to 411 inclusive. As is therein explained, an alloy suitable for fusion with a standard hard glass (Corning G-'71) may be formed by the alloyage of 54% iron, 28% nickel and 18% cobalt.

The cap 2 is provided centrally with an opening which permits the introduction of an anode supporting rod 3 which may suitably be composed of steel. The rod 3 is provided at its upper end with an outwardly projecting flange 4 which is 30 secured by a projection weld or some equivalent means to the outer surface of the cap 2. The anode itself is preferably made of graphite and is shown as comprising a heavy block 5 of that material screw-threaded onto the lower end of the rod 3. A connection to a suitable source of external potential (not shown) may be made through a section of stranded copper cable 3' swaged and soldered into the exposed end of the rod 3. The length of the latter is such that the {L0 anode projects well down into the tube toward the bottom of the envelope.

The lower end of the envelope l is closed by a circular metal header 6, peripherally sealed to the edge of the glass cylinder and having a flattened surface 8 projecting toward the interior of the envelope. In order toprovide suflicient stiffness to resist distortion due to the pressure differential between the inside and outside of the envelope the central portion of the header is reenforced by a heavy plate 1 brazed or otherwise secured thereto and consisting of steel or a similar material of good mechanical strength. Sufiicient flexibility to avoid breaking of the glass to metal seal by expansion of the header and reinforcing plate is obtained by providing the former with a reentrant bend near its outer edge. A solid iron block 9 fixed to the lower surface of the plate I afiords a convenient means of connecting the header to a potential source.

The lower end of the tube contains a pool ID of cathode material, suitably mercury, which covers the surface of the header and is adapted to serve both as a cathode and as a source of mercury vapor during operation of the tube. In connection with the cathode, I provide a make-alive device exemplified as an electrode l2 supported in permanent contact with the surface of the mercury pool. A particular form of electrode which has proven satisfactory for this use comprises a tapered cylinder of semi-conducting material, such as silicon carbide coated with a mixture of boron carbide and clay. Other types of cyclically operable make-alive devices may also be employed for this use.

In the arrangement shown, the starting electrode i2 is mounted on a reentrant thinible it which is provided at its lower end with an outwardly projecting fiange. This in turn is joined through an externally exposed glass fillet M to a cylinder l5 which depends from. the lower surface of the header. A lead-in conductor 56 sealed through the upper end of the thimble i3, makes it possible to connect the starting electrode H! to a source of control potential (not shown) and to render the tube conductive during a desired portion of each voltage cycle. Since ignition of the discharge takes place at the beginning of each cycle at the contact surface between the electrode 52 and the cathode material It), the cathode spot has no opportu nity to move far from the vicinity of the electrode.

When discharge devices of the character described are operated in circuits containing inductance there may be enough residual ionization holding over from the conductive half cycle to create a danger of arc-back during the initial moments of the non-conductive half-cycle. This danger can be considerably reduced by associating with the discharge path cooling means adapted to maintain a low vapor pressure in the neighborhood of the anode and to provide extensive metal surfaces in contact with the arc stream for hastening the deionization process. For example, one may use for this purpose a helical coil l9 suitably of seamless steel tubing extending from the surface of the cathode up to and around the anode. This coil is preferably of somewhat open configuration at the upper portion thereof so that heat radiated from the anode maintains the upper side walls of the envelope at a temperature not far below that of glass near the anode seal. The temperature equalization tends to decrease the danger of cracking the glass under extreme conditions of operation.

While the provision of cooled metal surfaces in contact with the arc is of definite value, it is an important aspect of my present invention that a much greater degree of insurance against areback is obtained by the use of transverse baffles arranged as described in the following. In this connection it is desirable that the anode be shielded substantially entirely from direct exposure to the cathode surface and that the discharge cur rent be caused to take a tortuous rather than a direct path.

In the particular arrangement shown in Fig. 1 the shielding means includes a transverse bafile 2i of relatively limited area interposed between the anode and the particular region of the oathode surface at which the discharge is initiated. This bafiie is constituted of a flat coil of tubing which is positioned directly above the makealive electrode. Its central portion, which would otherwise be somewhat open, is rendered substan tially impervious by the use of a pair of refractory metal disks 22 secured together by a pin or small bolt.

Above the bafiie, and thus relatively more remote from the cathode surface, there is provided a second transverse baffle comprising the circumferentially extending coil 2a. This is formed with a central opening which is preferably of somewhat smaler diameter than the coil 2! and is aligned with it.

These two baffle portions cooperate to shield the anode from the cathode end of the discharge device and to cause a certain deflection of the discharge current. The useful result obtained consists in a substantial increase in the arc-back resistance of the device. In some cases, this increase has been found to be of such magnitude that the tube may be safely operated at many times the voltage which is permissible in the absence of a bafile system.

While I do not wish to be bound to any particular explanation of the results thus obtained, I consider that they are due in large part to the exclusion of cathode material from the vicinity of the anode. Direct splashing of such material on to the anode surfaces is obviously prevented by the interposed bafiies. Indirect transfer of mercury vapor into the anode chamber is apparently greatly lessened by causing the discharge current to traverse a more or less complex baffle system such as that described.

In order to maintain the baffles 2i and 24 and the tubing 9 at a low temperature I consider it desirable to circulate a cooling fluid therethrough. A convenient manner of forming the inlet and outlet connections for such fluid is shown in the drawing wherein are illustrated outwardly projecting cylinders 26 and 2'5 which respectively surround corresponding apertures in the surface of the header. These cylinders are hermetically sealed to concentric pipes 28 and 29 by fusion welding the same at their outer contiguous edges. The fluid connections for the tubing l9 do not appear in Fig. l but are indicated in dotted outlines at 28' and 29 in Fig. 2.

The precise baffle construction which I have described in the foregoing is in no way essential to my invention, and in Fig. 3 I have illustrated a modified form which such bafiles may take. In this case the envelope is constituted of a pair of concentric metal cylinders 30 so spaced as to provide a water jacket between them. Within the envelope are provided an anode SI and a pool-type cathode 32, these being separated by a bafiie system.

The lower or central bafile is constituted of a spiral coil 33 corresponding generally to the coil 2| described in connection with Fig. 1. As appears more clearly in Figs. 4 and 5 the central part of this coil is rendered impervious by the use of a pair of metal disks 34 which are positioned on opposite sides of the coil and held in place by means of a bolt 35. The position of the baffle 33 is such that it lies directly above a starting electrode 36 which is provided in operative relation to the cathode 32. Thus the baffle is interposed directly between that portion of the cathode surface which constitutes the region of active discharge and the anode 3|. Asecond baflie 31, displaced from the first, is provided in the form of a relatively thick metal plate having an opening 38 at the center thereof. This plate is provided with a depending flange 39 which is peripherally secured to the inner surface of the envelope wall so as to be in good heat exchanging contact therewith. By this means the bafiie may be maintained at a temperature below that of the fusing temperature of the metal of which it is constituted. As far as operation is concerned, the baffle system just described is equivalent to that described in connection with Fig. 1 and serves to increase manifold the arc-back resistance of the discharge device.

While I have described my invention in connection with particular structural arrangements, it is clear that various other baffle combinations may be used without departing from the invention. I, therefore, aim in the appended claims to cover generically all such equivalent variations as fall Within the true spirit and scope of the foregoing disclosure.

What I claim as new and desire to secure by Letters Patent of the United States, is:

1. An electrical discharge dev'ce comprising a single main anode and a pool-type cathode spaced from the anode, means providing metal wall surfaces laterally surrounding the discharge path between the anode and cathode, a makealive device operable cyclically to initiate a discharge at a definite region of the cathode surface, thereby to avoid danger of the cathode spot becoming attached to the said metal wall surfaces, a first transverse bafiie positioned relatively close to the cathode surface and directly interposed between the said definite region of the cathode surface and the anode, said bafile covering only a fractional portion of the total area of the cathode so as to avoid excessive constriction of the discharge, and a second transverse bafile displaced from the first bafiie and cooperating therewith to shield the anode from direct exposure to the cathode and to assure a tortuous path for the discharge current, said second baflie being relatively more remote from the cathode surface than the first bafile.

2. An electrical discharge device comprising a single main anode and a pool-type cathode spaced from the anode, means providing metal wall surfaces laterally surrounding the discharge path between the anode and cathode, a make-alive device operable to initiate a discharge cyclically at the central region of the cathode surface, thereby to avoid danger of the cathode spot becoming attached to the said metal wall surfaces, a first transverse baifie positioned relatively close to the cathode surface and directly interposed between the said central region of the cathode surface and the anode, said baflle covering only a fractional portion of the total area of the cathode so as to avoid excessive constriction of the discharge, and a second baflle displaced from the first and extending circumferentially of the discharge chamber so as to provide a central opening therein, said opening being aligned with said first bafile so that the anode is effectively shielded from direct exposure to the cathode surface by the combination of battles, and means for supplying a. cooling fluid to the first bafile and to the said wall surfaces.

LAWRENCE D. MILES. 

